1. Technical Field
The present invention relates to a method of manufacturing a semiconductor device and to a wet processing apparatus.
2. Related Art
For integrating two kinds of transistors having different power source voltages (Vdd) into one chip, gate insulating films having different film thicknesses need to be formed on one and the same silicon substrate. Such a gate insulating film is formed by what is known as the multiple oxide technique. Hereafter, the procedure therefor will be described. First, a silicon oxide film having a large film thickness is formed on two transistor formation regions. Thereafter, the transistor formation region for forming a transistor (I/O or the like) having a high Vdd is covered with a resist, and the silicon oxide film formed on the transistor formation region for forming a transistor (core transistor) having a low Vdd is removed by etching with use of BHF or the like. Subsequently, the resist is removed, and a silicon oxide film having a small film thickness is formed on the transistor formation region for forming the low-Vdd transistor. This completes forming gate insulating films having different film thicknesses.
In recent years, in accordance with the miniaturization of devices, high uniformity and reduction of foreign substances are demanded in removing silicon oxide films by etching. For this reason, a technique for removing the silicon oxide film with use of the sheet-by-sheet method is now adopted. In the sheet-by-sheet method, a chemical solution ejecting nozzle is directed, for example, towards the center on the front surface of a substrate which is held and rotated at a predetermined rotation number, and a chemical solution such as BHF is ejected from the nozzle. This removes the silicon oxide film on the front surface of the substrate by etching.
Japanese Laid-open patent publication No. 10-79371 discloses a wet etching processing method of etching the front surface of a substrate by spraying an etchant onto the front surface, wherein the etchant is sprayed onto the back surface of the substrate as well simultaneously with the front surface. By this, the etchant is sprayed onto the back surface of the substrate simultaneously with the front surface, so that a liquid flow is generated on the back surface, thereby preventing circulation of the etchant to the back surface of the substrate with help of this liquid flow. For this reason, it is said that particles contained in the etchant flowing from the front surface of the substrate will hardly adhere to the surface of the substrate. Also, since the front surface and the back surface of the substrate are processed with use of the same etchant, the zeta electric potential generated on the front surface and on the back surface will be the same electric potential, so that the adhesion of the particles onto the back surface because of the electric potential difference can be prevented.
In the meantime, in removing the oxide film by etching with use of the sheet-by-sheet method, there arises a problem in that defects such as digging or melting of diffusion layers in the front surface of the substrate occur, thereby leading to decrease in the yield.
The present inventors have made eager studies on the cause of the generation of such defects. As a result, the inventors have found out that this is because of the following reason. At the time of removing the oxide film by etching, the substrate is electrically charged and, when a BHF chemical solution is brought into contact with a specific site (for example, the center) on the substrate, the electric charge accumulated in the substrate escapes all at once to the electrically conductive chemical agent side.
FIG. 10 shows a state in which BHF, which is an electrically conductive chemical solution, is supplied to the front surface of an electrically charged semiconductor substrate 1000, found by the present inventors. In a wet processing apparatus of sheet-by-sheet system, when the semiconductor substrate 1000 is mounted on a mounting base constructed, for example, with Teflon (registered trade mark) or the like, the back surface of the semiconductor substrate 1000 will be positively charged and, in accordance therewith, the front surface of the semiconductor substrate 1000 will be negatively charged. When an electrically conductive chemical solution is supplied to the front surface of the semiconductor substrate 1000 in a state in which the semiconductor substrate 1000 is electrically charged in this manner, the static electricity escapes from the front surface of the semiconductor device 1000 into the electrically conductive chemical solution to form an electric current path, whereby damage is generated on the front surface of the semiconductor substrate 1000.
Japanese Laid-open patent publication No. 2005-307311 discloses a substrate processing apparatus having a static electricity regulating unit and a wet-type processing apparatus. The static electricity regulating unit has a static electricity sensor that detects the electrically charged state of a substrate and a charged part blower, thereby to remove the static electricity that is present in the electrically charged substrate or to charge the substrate to an arbitrary electrically charged state. By this, it is held that, by suitably controlling the electrically charged state of the substrate in accordance with the kind of a wet-type process, deficiency caused by the static electricity on the front surface of the substrate can be restrained. However, with this construction, there is a need to provide a charged part blower, so that the apparatus construction will be large and complex.
Japanese Laid-open patent publication No. 2003-188138 discloses a step in which, in order to restrain the charging of the substrate to be processed, an electrically charged fluid for removing electricity whose charged amount can be controlled is supplied to a semiconductor wafer during the cleaning processing step, whereby the electric charge that is present on the front surface side of the semiconductor wafer is removed in the cleaning processing step. Japanese Laid-open patent publication No. 2003-188138 gives a description that pure water for removing electricity may be supplied to the back surface of the wafer before pure water for cleaning is jetted onto the front surface of the water.
FIG. 11 is a schematic view illustrating the conventional art disclosed in Japanese Laid-open patent publication No. 2003-188138. It is disclosed that when negatively charged pure water is allowed to act on a semiconductor substrate 1000 having a negatively charged front surface and a positively charged back surface, the positive charge on the back surface of the semiconductor substrate 1000 is neutralized, whereby the charged amount of the semiconductor substrate 1000 can be reduced. However, with such a method, it is difficult to control the timing for reducing the charged amount of the semiconductor substrate 1000 approximately to zero. Also, when the period of time for supplying the negatively charged pure water becomes long, there is a fear that the back surface side of the semiconductor substrate 1000 will be negatively charged, and the front surface will be positively charged. When the semiconductor substrate 1000 remains in an electrically charged state and an electrically conductive chemical solution is allowed to act on the front surface of the semiconductor substrate 1000, the electrically conductive chemical solution will be an electric current path, and it is not possible to prevent the damage from being generated onto the front surface of the semiconductor substrate 1000, in the same manner as shown in FIG. 10.
Also, even if the etchant is sprayed onto the back surface of the semiconductor substrate simultaneously with the front surface as disclosed in Japanese Laid-open patent publication No. 10-79371, it is not possible to prevent the damage from being generated onto the front surface of the semiconductor substrate.